1. Field of the Invention
The invention relates to a method for the production of a silica glass crucible with crystalline regions from a porous silica glass green body, and to the use of the method for pulling Si single crystals according to the Czochralski method (“CZ method”).
2. Background Art
During the production of Si single crystals by means of CZ methods, the quality of the quartz glass crucible that is used plays a decisive role in relation to the properties of the crystal to be produced, and in the relation to the maximum achievable yield of monocrystalline material. In the standard pulling process, quartz glass crucibles are used which are melted from crystalline SiO2 particles (for example quartz sand) in a melting process, generally in an arc discharge. In this case, a closed, amorphous, vitrified inner layer and a fully vitrified outer body with low porosity are formed. It is desirable that the inner layer contain the fewest possible amount of bubbles, and when present, in the smallest possible size. Impurities in the inner surface of the crucible, introduced during the production process or which diffuse into the crucible surface from the starting material during crystal production by the subsequent CZ method, lead to corrosion of the inner surface during crystal production. Corrosion, in the case of amorphous quartz glass crucibles, is a limiting factor for crucible life within which it is possible to produce monocrystalline material.
These corrosion phenomena are avoided by applying materials that cause near-surface crystallization of the amorphous vitrified layer during the CZ method. One such method is described, for example, in European patent EP 0 753 605 B1, which further indicates that the mechanical stability of the quartz glass crucible can be increased if crystallization of the outside of the crucible is also induced. The disadvantage of the methods described in EP O 753 605 B1 is that the chemicals which induce crystallization are applied to the amorphously vitrified inner surface of a crucible or, in the case of coating the outside of the crucible, to a vitrified body with low porosity. The crystalline quartz layer then created in the CZ method reaches only a thickness of less than 1 mm when coating the inside of the crucible, and less than 2 mm when coating the outside of the crucible. This means that the increase in stability of the crucible when coating the outside of the crucible is unduly limited.
When coating the inside of the crucible, the very thin crystalline layer that is formed in the CZ method leads to mechanical stresses between the crystalline and amorphous regions of the quartz glass crucible. These are due to the differing thermal expansion coefficients and the differing mechanical stabilities of the amorphous and crystalline modifications of the crucible material as a function of temperature. As a result of these stresses, quartz particles may be displaced from the inner surface of the crucible and, via the Si melt, reach the growing crystal where they induce undesired dislocations. Furthermore, under the thin crystalline layer of the inner surface of the crucible, the bubbles present in the amorphous starting material can grow at an unreduced rate during the CZ method and, when displaced, likewise contribute to the emission of quartz glass particles into the Si melt.
It is not possible to increase the thickness of the crystalline layer by introducing substances into the material being processed which promote subsequent crystallization of the amorphous quartz crucible by means of known melting methods to form a quartz glass crucible, since the crucible begins to crystallize during production (when melting). Upon cooling, cracks are formed which make the crucible unusable, for example due to the β to α cristobalite transformation known in the literature.